1. Field of the Invention
The present invention relates to a proton conductive hybrid material which comprises a proton-conductive inorganic nanoparticle and a proton-conductive polymer, and which has small-regulated particle diameter. In particular, the present invention relates to a catalyst layer for use of a fuel cell, wherein the proton conductive hybrid material, which has small-regulated particle diameter, is included in primary pores of the catalyst layer, and to a method for producing the material and the catalyst layer.
2. Description of Related Art
In recent years, much attention has been paid on the development and use of novel organic-inorganic nanocomposite materials as proton conductors, especially because of its possible commercial application in polymer electrolyte membrane fuel cells (PEMFC), since these materials can show high conductivity and can form thermally stable membranes at elevated temperatures.
Considering the properties of proton conductive materials, in general, polymeric materials show a desirable conductivity under humidified condition; however, the disadvantages are its degradation at higher temperatures and its high resistance when it is not humidified properly. On the other hand, inorganic conductors are thermally and mechanically more stable, but they are brittle and the pure oxides do not show high proton conductivity. However, its ability to retain water even at high temperatures makes it attractive as far as the proton conductivity is concerned. Hence, Proper methods should be made to enhance the proton conductivity of these materials.
There is a general agreement in the scientific community that there is a need for a novel proton conducting material which can show appreciable conductivity at elevated temperatures along with other properties. Synthesis of proton conducting inorganic-polymer hybrid material seems to be an option in order to overcome the difficulties of Nafion®, and therefore can be used in PEMFC operating at higher temperature. In hybrids, the thermal stability is provided by the inorganic back bone, while the polymer part confers the required specific properties such as flexibility and processability.
PEFC for automobiles requires continuous operation from −25° C. at initial motion to 120° C. during operation. Fluorine-based ionomers, a major example of which is Nafion®, show favorable proton conductivity at low temperatures, but cannot be used at high temperatures because of the sudden decrease in proton conductivity and poor heat resistance. In addition, fluorine-based ionomers require high production cost, and place a heavy load to the environment because of the fluorine component. An alternative material, SPES, which is a hydrocarbon-based ionomer, is inexpensive and provides a high structure controllability, oxidation resistance, and heat resistance, but it requires water activity to show high proton conductivity (see Non Patent Document 1). Therefore, the hydrocarbon-based ionomer cannot maintain proton conductivity at high temperatures and under normal pressure. Development of proton conductivity in a wide temperature range requires structuring a hybrid structure material. Many studies have been made on modification of hybrid materials with inorganic materials which can maintain proton conductivity at high temperatures (heteropoly acid, hydrogen sulfate, and inorganic solid proton conductors) (see Non Patent Document 2). Hydrated zirconia and a tetravalent metal described herein can maintain proton conductivity at high temperatures. In addition, hydrated zirconia is a precursor of ZrS and ZrP which show high proton conductivity. These studies are mainly focused on ionomers as electrolyte membranes, and few studies discuss about ionomers as electrodes. In an electrode, a catalyst layer composed of carbon as the electron transfer pathway, an ionomer as the gas or proton transfer pathway, and a platinum catalyst as the electric power generation site has, as shown in FIG. 1, a complicated structure including primary pores (20 nm to 40 nm) and secondary pores (40 nm) of agglomerate size. The primary pores have a small diameter and thus inhibit the entrance of a polymeric ionomer, which hinders the formation of proton pathways extending to the inside catalyst. Therefore, the total utilization of the catalyst is on the order of only 20%. Therefore, the size of the ionomer in the electrode is important. In spite of the background of the electrode structure, the structure analysis of electrodes composed of ionomers, or the performance evaluation of fuel cells including such electrodes have not been conducted.
Non Patent Document 1: Cotter, C. J. Engineering Plastics: A Handbook of Polyaryleters; Gordon & Breach: London, 1965.
Non Patent Document 2: Li, Q. He, R. Jensen, J. O. Bjerrum, N. J. Chemistry of Material 15 (2003), 4896-4915.